Anguilliform leptocephali from a fixed station in Sagami Bay, Central Japan

Abstract

Anguilliform Leptocephali from a Fixed Stationin Sagami Bay, Central JapanMasaki Miya1 and Masumi Hirosawa2,31 Natural History Museum and Institute, Chiba, 955-2Aoba-cho, Chuo-ku, Chiba 260, Japan2 Department of Oceanography, Tokai University, 3-20-1Orido, Shimizu, Shizuoka 424, Japan3 Present address: 5-20-3 Hiratsuka, Kanagawa 254, Japan(Received September 22, 1993; in revised form December 14,1993; accepted December 21, 1993)The anguilliforms are currently represented by 21families and 153 genera, with about 720 species(Castle, 1984). Their larvae, commonly known asleptocephali because of their slender heads and leaflike bodies, are oceanic, being presumed to bespawned and hatched offshore in clean, warm, salinewater, even those of adults that spend most of theirlives in estuaries or fresh water (Smith, 1989b).Although numerous papers have dealt with "lepto-cephalus taxonomy" since the mid 19th century, dueprimarily to the distinctiveness of the larval forms(for review, see Smith, 1989b), there still remain anumber of leptocephali that cannot be adequatelyidentified to species. In general, the biology ofleptocephali is poorly known, except for the spawning grounds and spatial and temporal distribution,for example, of several commercially important ang-uillid species (for review, see Smith, 1989a; Tsuka-moto, 1992) and a few other abundant, easily identified species such as the moringuid, Moringua edwar-dsi (Castle, 1979). Apart from difficulties in identification, one factor that hinders further knowledge oftheir biology stems from the fact that although studies with wide geographic coverage have been numerous, few have been intensive and site-oriented,including repeated collections made in a consistent,quantitative manner.Since 1968, a total of 134 deep-sea oblique-haulsamples have been collected at a fixed station inSagami Bay (35°00'N, 139°20'E, ca. 1,500m depth;Fig. 1) over a twenty-year period, in which eachmonth of the year was covered. The station waslocated near the center of Sagami Bay, where about20 adult anguilliform species are so far known(Yamada, 1990; Kudo and Okabe, 1991; Yamadaand Kudo, 1992; Miya, unpublished data), includingFig. 1. Location of the fixed station in Sagami Bay(O)- Depth contours in meter.several commercially important ones such as Congermyriaster (reliable catch statistics unavailable). Although the samples yielded only about 50 leptocephali, a number comparable with that from just severalcatches from more oceanic water adjacent to the bay(Miya, unpublished data) or from Kuroshio waters(Uematsu et al., 1990), information on the seasonaloccurrences of relatively more abundant leptocephali, together with other "negative" data, willguide future studies on Sagami Bay leptocephali.Materials and MethodsStudy site.—All samples were collected at a fixedstation near the center of Sagami Bay (35°00'N,139°20'E, ca. 1,500 m depth; Fig. 1) over a twenty-year period (1968-1988). Sagami Bay is locatedabout mid-way along the length of Japan, its ocean-ographic features, particularly those of the surface,being greatly affected by the Kuroshio current,which flows northward along the eastern coast ofJapan. A branch of the Kuroshio flows north-northeasterly into the bay, usually through theOshima-West Channel and subsequently passesthrough the Oshima-East Channel on its return tothe Pacific Ocean, thereby sweeping the southernhalf of the bay (Uda, 1937; Nakataet al., 1989). Thesurface temperatures vary considerably over a year,ranging from 15 to 25°C (see fig. 2 in Miya andNemoto, 1991). The coolest surface temperaturesare recorded during February and March, considerable vertical mixing occurring between the surfaceand 120 m depth, and the warmest, during Augustand September, when a clear, seasonal thermocline— 68 —